XBMC offers instructions on how to do the install on its wiki. You just need to enter a few commands in the command line, reboot, and you'll have that kickass, open-source media center running on your Apple TV 2. Unfortunately, ATV3 owners, your Apple TV still isn't supported.

In case you missed it, you can see what's new in the Frodo 12 release, which this update was built upon, here.